1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device, and more particularly to a method for manufacturing a liquid crystal display device that employs a plastic substrate to make a liquid crystal panel lighter and thinner.
2. Description of the Related Art
In terms of actual use of a liquid crystal display device, particularly a portable information terminal device or a portable telephone employing the liquid crystal display device therein, one of critical problems to be solved is to make a liquid crystal panel light-weighted and thin.
Efforts have been vigorously made to overcome such problems by reducing the density of a glass substrate or by making the thickness of the glass substrate thin. When glass density is reduced, a silicon dioxide (SiO2) mainly constituting a glass decides substantially all physical property values of glass and hence, a technique to further lower the glass density faces its limit, resulting in difficulty in further reducing the glass density. On the other hand, when the plate thickness is made thin, the strength of the glass substrate is drastically degraded so that facilities in a manufacturing line are to forcibly be changed to a large extent and further, the physical resistance against external forces becomes lowered. Accordingly, it is said that the thickness of the glass substrate can at most be made 0.4 to 0.5 mm as its minimum value.
As a liquid crystal panel achieving lightness and thin structure thereof, a so-called simple-matrix-type monochrome liquid crystal panel not employing active elements such as a TFT (Thin Film Transistors) for driving liquid crystal is exemplified. In such a liquid crystal panel, material which constitutes a matrix is formed of transparent electrode material referred to as ITO and is formed at a relatively low temperature and hence, it is possible to use a plastic substrate or the like as a base substrate on which the transparent electrode material is formed. Actually, the plastic substrate having such construction has been used in the portable information terminal device and the portable telephone.
As for a trend of the liquid crystal display panel, there has been observed a phenomenon that an amount of information which can be processed as display information has been increasing in accordance with improvement in performances of the recent portable terminal equipment and then, the image to be displayed has been changing from a monochrome one and a still picture to a color one and a motion picture, respectively.
However, the simple matrix-type liquid crystal display device employs an STN (Super Twisted Nematic) liquid crystal mode as a liquid crystal driving mode and hence, the simple matrix-type liquid crystal display device has the following disadvantages. That is, comparing with an active matrix type liquid crystal display device, which employs a TN (Twisted Nematic) liquid crystal mode and TFTs, and is used in a notebook type personal computer or a monitor, both being popularly and widely used, the simple matrix-type liquid crystal display device does not provide a user with sufficient quality of images to be displayed, so that gray-scale display deteriorates or the user feels residual images
In consideration of the above-described problems, the following techniques have been developed and are disclosed such as in Japanese Laid-open Patent Publication 212116/1999 and Japanese Laid-open Patent Publication 116158/1997. That is, active elements such as a TFT element are formed on a plastic substrate. In the former publication, the active elements are formed on a glass substrate and then, a plastic substrate is attached to the surface of the glass substrate on a side thereof on which the elements are located, and further, a portion of the glass substrate on a side thereof on which the elements are not located, is polished in a direction of the thickness of the substrate to remove the glass substrate. In the latter publication, active elements are formed on a plastic substrate or a glass substrate having a thickness of not greater than 0.5 mm. However, many technical problems to be solved still have been found in a device employing a plastic substrate and further, such device requires expensive facilities, thereby preventing the device from being put into practical use.
In a liquid crystal display device having color filters together with active elements on a plastic substrate, the plastic substrate on which the color filters are formed exhibits the poor resistance against heat and solvent and hence, pigments and solvents available for use as a color filter are limited, whereby the color reproducibility cannot sufficiently be achieved. Accordingly, development of a liquid crystal display device having the following construction and advantages has been strongly required. That is, a liquid crystal display device is formed lighter and thinner, and further formed to operate in the same manner as in a case where a color and active matrix liquid crystal panel operates in a TN liquid crystal mode.
Furthermore, even in a case where a plastic substrate is employed to make a liquid crystal display device lighter and thinner, the plastic substrate has problems other than poor resistance against heat and solvent in that the plastic substrate is easily deformed and therefore, it is so difficult to process the plastic substrate in steps for manufacturing a liquid crystal panel compared to the case where a glass substrate is processed.
Accordingly, it is an object of the present invention to provide a method for manufacturing a liquid crystal display device in which a TFT substrate is employed and a liquid crystal can be formed simultaneously lighter and thinner.
A method for manufacturing a liquid crystal display device according to the present invention is constructed as follows. That is, the method comprises:
a step for disposing a TFT substrate and a counter substrate facing the TFT substrate apart a predetermined distance from each other; and
a step for filling a space between the TFT substrate and the counter substrate with a liquid,
in which the liquid crystal display device is constructed such that at least one of the TFT substrate and the counter substrate consists of a plastic substrate, the method further being constructed such that the method comprises a substrate separation step for cutting and dividing a combined substrate consisting of the TFT substrate and the counter substrate, both substrates interposing the liquid crystal therebetween, into substrate units, and one of surfaces of the plastic substrate is supported by a support substrate until the combined substrate is cut and divided.
As described above, by using a plastic substrate as one of two substrates that constitute a liquid crystal panel in a method for manufacturing a liquid crystal display device, compared to a case in which a glass substrate having a thickness equal to that of the plastic substrate is used in a liquid crystal panel., the liquid crystal panel can be made light-weighted. Furthermore, in a case where the plastic substrate is made to have the same resistance against collision as that of the glass substrate, the thickness of the plastic substrate can be reduced compared to that of the glass substrate, thereby allowing a liquid crystal panel to have a thinner thickness.
The above-described method has the following detailed construction. That is, a method for manufacturing the TFT substrate comprises:
a step for forming a thin film transistor and a wiring on a first substrate;
a step for depositing a protective film covering the thin film transistor and the wiring on the first substrate;
a step for forming a color layer corresponding to the thin film transistor and a black matrix shielding light from being irradiated onto the thin film transistor on the protective film;
a step for depositing a flattening film covering the color layer and the black matrix on the protective film;
a step for opening a part of the protective film and the flattening film to form a contact hole reaching a source electrode of the thin film transistor in the protective film and the flattening film;
a step for forming a transparent pixel electrode covering the contact holes and connected to the source electrode on the the flattening film;
a step for forming a spacer on the flattening film;
a step for forming an orientation film covering the transparent pixel electrode and the spacer on the flattening film, and subsequently, rubbing the orientation film;
a step for forming a seal material to surround a predetermined region on the orientation film; and
a step for dropping a liquid crystal into the region surrounded by the seal material constituting the orientation film, and
a method for manufacturing the counter substrate comprises:
a step for covering a second substrate with a transparent electrode and adhering a support substrate to a surface of the second substrate, the surface being located on a side of the second substrate opposite to the transparent electrode; and
a step for forming a material for an orientation film on the transparent electrode, and sequentially, rubbing the orientation film, and
the method for manufacturing a liquid crystal display device further comprises:
a step for disposing the TFT substrate and the counter substrate overlapping each other such that the orientation films of the TFT substrate and the counter substrate face each other, and then, making the orientation film of the counter substrate contact the spacer and the seal material of the TFT substrate;
a step for curing the seal material to make the TFT substrate and the counter substrate adhere to each other, thereby forming a semi-completed panel;
a step for removing the second substrate from the support substrate,
a step for cutting and dividing the semi-completed panel into panel units; and
a step for attaching a polarizer to a surface of at least the first substrate out of the first and second substrates constituting the panel unit, the surface being located opposite to the liquid crystal, and
the support substrate being previously attached to a surface of the second substrate, the surface being located opposite to the transparent electrode, before the step for forming the transparent electrode on the second substrate.
Furthermore, the above-described detailed method has the following additional construction. That is, the polarizer is attached only to the surface of the first substrate in the step for attaching a polarizer to a surface of at least the first substrate out of the first and second substrates constituting the panel unit, the surface of at least the first substrate being located opposite to the liquid crystal, and the second substrate is formed of a polarizer-cum-plastic substrate.
As described above, employing a polarizer-cum-plastic substrate as the second substrate out of the first and second substrates makes it possible to form a second substrate thinner than that could be achieved by use of a glass substrate and thereby form a liquid crystal panel further thinner.